It is often necessary to separate particulates from air or other gaseous fluids in which the particulates are entrained. For example, aerosols comprising small droplets of liquid dispersed into air are not easily analyzed unless the aerosol materials are separated from the air to produce a more concentrated sample that can then be analyzed. The aerosols or particulates can be liquids, solids, or semi-solids that are sufficiently small to be dispersed within and carried about in air and may include inorganic or organic chemicals, or living materials, e.g., bacterial cells or spores. Also, solids or semi-solids can be introduced into a liquid that is then dispersed within air as an aerosol mist so that the solids are carried within the liquid droplets comprising the aerosol mist.
Generally, it is difficult to identify materials comprising particulates entrained in a gaseous fluid unless the particulates can be collected by separating them from the air or other gaseous fluid and concentrated in a specimen suitable for analysis. In addition to identifying the type of particulates entrained in a gaseous fluid, it may be important to classify the size of the particulates. For example, when checking stack emissions, it is usually important to determine the materials carried as particulates within the emissions and the size of the particulates to determine whether the emissions conform to pollution control limits.
Particle impact devices are commonly used for collecting particulates from gaseous streams in which they are dispersed. Conventional particle impactors employ circuitous paths with many abrupt changes of direction along the passages through which a particulate laden fluid flows. The particulates, being substantially more massive than the molecules of the fluid in which they are entrained, fail to negotiate the abrupt turns in these passages and are thus separated from the moving fluid stream, collecting on the surfaces that they impact. To function properly, such prior art particle impactors require that the gaseous fluid stream be moved through the impactor at least at some minimum velocity. Typically, a separate fan is used to provide the required velocity to the fluid flowing into the particle impactor. One problem with such particle impactors of this type is that it is often difficult to separate the particulates collected by such particle impactors from the surfaces on which they have impacted. Furthermore, many of the particulates do not collect on the desired surfaces and are therefore unavailable for analysis and evaluation.
Another type of prior art particle impactor includes a rotating arm that is placed with flow of a fluid in which particulates are entrained. A separate fan is employed to move the fluid into the vicinity of the rotating arm. The particulates impacting the rotating arms are separated from the fluid. However, such collectors do not provide a simple and efficient mechanism to remove the particulates from the rotating arms.
Virtual impactors are another type of prior art device used for separating particulates from a gaseous fluid, again using the differences in mass of the particulates and the fluid molecules to facilitate the separation process. In this type of device, the gaseous fluid is directed along a passage and separated by a divider disposed within the passage into a fast moving major stream and a much slower moving minor stream. The more massive particulates remain in the slower moving minor stream, while the fluid and very small particulates continue through the device in the major stream. However, virtual impactors simply separate the streams, but do not provide a specific collection mechanism for separating the particulates from the fluid in which they are carried. Again, a separate fan or blower mechanism is employed to impart the required velocity to the fluid moving through the virtual impactor.
Although other types of particle impactors and virtual impactors are described in the prior art, none of them employ a single element for both moving the gaseous fluid in which particulates are entrained and providing a specific surface on which the particulates are collected as a result of their impact with that surface. Further, none of these prior art devices provide an efficient mechanism for actually collecting a concentrated specimen comprising the particulates separated from the gaseous fluid in which they were entrained. Clearly, it would be desirable to produce a compact particulate impactor for use in a portable device designed to collect a specimen of the particulates and to facilitate identification and analysis of the particulates carried by the gaseous fluid. Such a device should be capable of directly moving the gaseous fluid and particulates into the collector at a required velocity without use of a separate fan or blower, and provide a specimen containing the particulates once they have been separated from the gaseous fluid stream by impacting on a surface. It will be apparent that a miniaturized particle impactor of this type would be of considerable value in portable, hand-carried field apparatus used, for example, to identify bacteriological or chemical warfare agents that have been dispersed as an aerosol. Prior art particle impactors do not provide these features and functions.